Thermostatic expansion valves have previously been used in refrigeration systems using traditional refrigerants, such as R134a, R404A, R407A, R407B, R407C and R410C. In this case the expansion valve will typically function as a throttle valve regulating the superheating at the outlet side of the evaporator. This is typically done by means of a sensor positioned at the outlet of the evaporator. When an expansion valve is used in a CO2 refrigeration system, it still functions as a throttle valve as described above. However, in this case it does not regulate the superheating at the outlet side of the evaporator. Instead it regulates the pressure in the heat emitter, also in some cases called the gas cooler.
During the pressure regulation it will normally be attempted to obtain an optimal coefficient of performance (COP). When a CO2 refrigeration system runs supercritically, a specific pressure in the heat emitter is in principle coupled to each combination of evaporation temperature in the evaporator and outlet temperature of the heat emitter in order to obtain optimal COP. The pressure of the heat emitter is normally regulated by means of the thermostatic expansion valve and a sensor being filled with a gas/liquid mixture. This sensor functions like the sensor of a traditional system, i.e. the measured temperature at the outlet of the heat emitter is transformed into a corresponding pressure which is used for shifting a closing element in the valve. U.S. Pat. No. 5,890,370 discloses further details relating to obtaining optimal COP.
In prior art expansion valves for refrigeration systems using traditional refrigerants there is provided a sensor chamber and an evaporation pressure chamber. These chambers are separated by a diaphragm. In order to allow the valve to function properly, it must be ensured that the forces acting on the diaphragm from either side, i.e. the forces arising due to the pressure in the two chambers, must be of comparable size, e.g. of the same order of magnitude. Since both pressures act on equal areas of the diaphragm this means that the two pressures must be of comparable size. This is normally not a problem in refrigeration systems using traditional refrigerants, since the pressure of the refrigerant in such systems during operation is normally relatively low, typically approximately 1-12 Bar, though in some situations the valve may be subject to pressures up to approximately 42 Bar. It is well known to construct a system in which such pressures are present in a cost effective way.
However, when a refrigeration system is desired which uses a high pressure refrigerant, such as CO2, the pressure of the refrigerant will normally be as high as approximately 60-90 Bar. This puts strict requirements on the durability, strength and thickness of material of the valve, in particular of the diaphragm and parts surrounding the diaphragm, such as the walls of the sensor chamber and the evaporation chamber, thereby increasing production costs and making the manufacturing process more difficult.